Apparatus for controlling the speed of electric motors



C. T. EVANS March 3, 1936.

APPARATUS FOR CONTROLLING THE SPEED OF ELECTRIC MOTORS Filed Jan. 11,1954 INVENTOR ATTORNEY Patented Mar. 3, 1936 UNITED STATES PATENT OFFICEAPPARATUS FOR CONTROLLING THE SPEED OF ELECTRIC MOTORS Charles T. Evans,Jersey City, N. J., assignor to Air Reduction Company, Incorporated, New

York, N. Y., a corporation of New York This invention relates toapparatus for controlling the speed of electric motors.

Centrifugal governors have been successfully used with electric motorsof small size to maintain the motor speed very nearly constant. Suchgovernors have switch contacts, one of which moves away from the otherto open the circuit through the governor when the motor speed exceeds agiven value, and moves back again to close the circuit as soon as thespeed drops below the desired value. The circuit through the governor isopen and closed at very short intervals. I

In order to prevent the switch contacts from being destroyed by constantsparking, a condenser is connected across the governor. A smallercondenser can be used if a high resistance shunt circuit carries aportion of the motor current around the governor. Such a circuit reducesthe amount of current flowing through the a governor, and while somepower reaches the motor through this shunt circuit even when thegovernor switch is open, this power supplied through the resistance isnot enough to maintain the motor speed. The opening of the governorcircuit therefore causes the motor to slow down.

Small motors equipped with these governors are frequently connected withindustrial power lines which supply large motors and other loads likelyto cause surges in the power line. It may be that some power surgesoriginate in the motor circuit instead 'of in the line which suppliesthe power for the motor. These surges, whether produced by the loads onthe line or other causes, are often severe enough to puncture thedielectric of the governor condenser and cause a short circuit throughthe condenser. Particularly heavy power surges may also injure otherparts of the 40 governor circuit.

Such a breakdown of the condenser makes the governor ineffective tocontrol the motor speed because current flows to the motor through thecondenser when the circuit through the governor 45 is open. The speed ofthe motor becomes variable and uncontrolled. When the motor furnishesthe driving power for an operation requiring a substantially uniformmotor speed, as in torch cutting or welding, the work is very likely tobe ruined in the event of a failure of the governing action. With thegovernor ineffective, the mechanism driven by the motor is without speedcontrols, and in mills or factories this means that production is heldup until repairs 55 can be made. When repair parts are not imtrollingapparatus have proved not only inconvenient but very costly.

It is an object of this invention to provide improved apparatus forcontrolling the speed of electric motors efficiently and reliably inspite of surges'in the power line.

Another object is to provide particularly simple, inexpensive, andeffective means for nullifying the effect of power surges on the speedcontrol apparatus of an electric motor.

The principal advantage of the invention is the elimination of governorfailures resulting from injury to the circuit by power surges. Anotheradvantage is that this invention permits the use of less expensive andmore compact condensers because a lower factor of safety for thecondenser is practical.

Other objects, featuresand advantages of the invention will appear or bepointed out as the description proceeds.

In the accompanying drawing, forming a part hereof:

Fig: 1 is a view, mostly in section, showing an electric motor withspeed control apparatus embodying the invention;

Fig. 2 is an enlarged sectional view on the line 2-2 of Fig. 1; and

Fig. 3 is a wiring diagram for the apparatus shown in Fig. 1.

The drawing shows the invention applied to a series-wound motor ill, butthe invention is applicable also to other types of motors.

The motor l0 has a frame l2 with a bracket M for connection with amachine or other support. The armature shaft extends at one end into aspeed reduction unit l6 through which the motor operates a drive shaftI8.

In order to hold the motor speed substantially uniform at any value andto make it possible to adjust the speed instantly and accurately, themotor I0 is equipped with a governor 20. This governor has a plate 22 ofinsulating material-secured to the upper end of the motor armatureshaft. Two concentric conductor rings 26 and 28 are fastened to theunder side of the plate 22, and these conductor rings are contacted bybrushes 30 and 32, respectively.

Two brackets. and 38 are fastened to the top of the plate 22 and areelectrically connected with the conductor rings 26 and 28, respectively.Spring blade arms 40 and '42 are secured to the brackets 36 and 38 andhave contact points at their ends which normally touch each other.

The movement oi the contact point end oi, the arm 62 in one direction islimited by a link l 'l connected to a bell-crank lever it which ispivotally supported by the plate 22. A speed-adlusting screw 68 threadsthrough the top of the governor casing and bears against one arm of thebell-crank lever 46-. The screw 68 has a knob 56 on its upper end forconvenient manual adjustment of the governor.

When the motor is running, the contact ends of both arms 40 and 42 tendto move away from the center of rotation by centrifugal force. The endsof the arms t0 and 42 which are secured to the brackets 36- and 38 arefixed with respect to the plate 22, and the arms must therefore bend asthe contact points move away from the center of rotation. These armsbeing of spring material permit the contact points to move further asthe speed of the motorincreases and the centrifugal force becomesgreater. When the arm 42 moves out as far as the link M will permit, afurther increase in motor speed causes the arm 40 to move away from thearm 62 .and break the circuit through the governor. The arms All and 42thus comprise a switch that is opened bycentrifugal force at a certainmotor speed. Centrifugal governors of this type are well known in theart.

In order to-lessen wear on the contact points of the governor switch, acondenser 55 is connected across the brushes 3d and 32. Wear on thecontact points is further reduced by also connecting a resistance Edacross the brushes and in parallel with the condenser 55. The resistancetil serves as a by-pass' or shunt through whicha fraction of the currentto the motor flows around the governor. Thus, the governor switch servesas a conductor for only a part of the total current supplied to themotor.

An electrical discharge device comprising a tube 65, containingelectrodes in an atmosphere of neon, is connected across the condenser55. The tube 55 may be similar to that shown in Patent 1,511,744, datedOctober 14, 1924;. During the ordinary operation of the circuit nocurrent flows through the tube 65, but any surges of a potential likelyto injure the condenser or other parts of the circuit will dischargethrough the gas in the tube. The gas pressure in the tube 65 and thespacing of the electrodes are such that the breakdown voltage of thetube, that is, the voltage necessary to start a discharge through thetube, is well below the maximum voltage which the condenser can standwithout danger of injury.

The motor it is series-wound and the circuit includes field coils it, itand brushes l6, l5. Current flows irom a power line to the field coilEl. From this coil ii the current flows through parallel circuits to themotor brush E i. One of these parallel circuits is the resistance til,and the other parallel circuit includes governor brush 32, conductorring 28, bracket to, arms 36 and d2, bracket 36, conductor ring 26, andgovernor brush 38.

From the motor brush it current flows through the motor armature, motorbrush it, and field coil 32, back to the power line. The condenser 55prevents any substantial arcing at the contact points of the governorswitch when the circuit is broken. The motor it can be operated oneither direct or alternating current.

Surges occur in themotor circuit and these surges are sometimes ofsufiicient potential to break down the condenser insulation andestablish a short circuit through the condenser so that the governorswitch is no longer efiective to control the motor. speed. Whether thesesurgeseflect can be nullified by the tube connected across the condenserand designed to provide a shunt circuit for all power of a voltage highenough to break through the condenser insulation.

One embodiment of the invention has been illustrated and described, butchanges and modifications can be made without departing from theinvention defined in the claims.

I claim:

1. The combination with an electric motor of a governor including aswitch for opening and closing a circuit of the motor to control themotor speed, a condenser connected across the governor switch to preventsparking at the contact points when the circuit through the governorswitch is broken, an electric discharge device connected across thecondenser and adapted to discharge current when the voltage across thecondenser and switch reaches a given value which is less than the safemaximum voltage oi? said condenser, and a resistance connected inparallel with the condenser and. governor switch, and serving todischarge the condenser and provide a by-pass for current around thegovernor switch so that only a portion of the current flowing to themotor passes through the governor switch.

2. The combination with acentrifugal governor having a switch for makingand breaking a motor circuit, a condenser connected in the circuit withthe governor switch toprevent arcing at the contact pointswhen saidswitch opens to break the circuit, a gas-filled electric dischargedevice connected in parallel with the condenser and having spacedelectrodes between which current flows to partially discharge thecondenser when the voltage across the condenser exceeds a given valuewhich is substantially greater than that of the inductive energy in thegovernor circuit when the switch opens, but less than the maximum safevoltage of said condenser, and a resistance connected in. parallel withthe condenser irom-heavy power surges in the governor 4. In combinationwith a governor switch for making and breaking a motor circuit; acondenser connected in parallel with the governor switch to preventarcing at the contact points of said switch; and a gas-filled electricdischarge device connected in parallel with the governor switch and thecondenser, said discharge device having spaced electrodes between whichcurrent flows if the voltage across the device exceeds a given value,whereby power surges of dangerous magnitude are by-passed around thecondenser and the governor switch. I

5. The combination with an electric motor, of a centrifugal governorhaving a switch adapted to operate at short intervals to open and closea circuit to control the motor speed, a condenser in the governorcircuit to prevent the inductive energy of the circuit from arcingbetween the contacts of the switch as said switch op ns to governorswitch for automatically opening and closing a circuit between the motorand a power line, and a condenser connected in the governor switchcircuit and of a capacity to absorb the normal inductive energy 01' themotor to prevent.

sparking when the switch is opened, of a discharge device connectedacross the condenser, said device comprising a neon-filled tubecontaining electrodes which are spaced to provide a discharge gap wideenough to prevent the flow of power through the tube during the ordinaryoperation of the circuit, the neon pressure and electrode spacing beingof such'values that a discharge through the tube will occur when thevoltage of the governor circuit rises to a value substantiallygreater'than the line voltage and less than that which will break downthe condenser or cause other injury to the governor circuit.

7. An electric circuit including a motor governor which opens and closesa motor circuit; a

condenser of substantial capacity connected across the governor toabsorb the inductive energy of the motor and prevent arcing when on thegovernor circuit.

breaking the circuit; and a gas-filled discharge device connected inparallel with the condenser and governor to provide a circuit forelectricity when the voltage 'of the governor circuit rises to thebreakdown voltage of the discharge device.

8. The combination with an electric motor or a centrifugal governorincluding a switch for opening and closing a circuit of the motor tocontrol the motor speed; a condenser connected in the governor switchcircuit and adapted to prevent sparking at the switch contact points;and a gas-fil1ed electric discharge device connected in said governorswitch circuit in position to protect both the governor switch andcondenser from power surges of excessive voltage.

9. The combination with an electric motor of a centrifugal governorincluding a switch for opening and closing a circuit of the motor tocontrol the motor speed; a condenser connected in the governor switchcircuit in position to prevent sparking at the switch contact points;and a neon-filled tube containing spaced electrodes, the pressure of theneon and the gap between the electrodes being of such values that thebreakdown voltage of the tube is greater than the voltage induced in thegovernor switch circuit when the switch opens, but substantially lessthan the maximum voltage which can be safely impressed CHARLES T. EVANS.

